(i) Field of the Invention
The present invention relates to a distributed antenna system, a distributed antenna switching method, a base station apparatus and an antenna switching device, and more particularly to a distributed antenna system, a distributed antenna switching method, a base station apparatus and an antenna switching device for making the communication using an optimal antenna efficiently in accordance with a traffic situation in a radio communication system in which a number of antennas are geographically distributed.
(ii) Description of the Related Art
In a radio communication system represented by a portable telephone, with the emergence of a smart phone, there has been a growing demand for higher speed communications in the data communication system, providing the high speed radio infrastructure for a Long Term Evolution (LTE) method or Worldwide Interoperability for Microwave Access (WiMAX) method that is the next generation high speed radio communication method. In the conventional portable telephone system, a macro cell method for covering the wide area with one base station apparatus is adopted. To improve the radio environment inside the building to which the electric wave is difficult to reach in a macro cell, a Distributed Antenna System (DAS) for distributing the antennas inside the building is well known. In the conventional distributed antenna system, a method for routing a leakage coaxial cable from the base station apparatus to radiate the electric wave around the cable laid down is well known. Also, a method for distributing an analog transmission signal inputted or outputted by the base station apparatus via a device for multipoint connection to plural antennas along the coaxial cable is well known. In the conventional distributed antenna system, the input/output signal of the base station apparatus is distributed to plural antennas, whereby the same signal is inputted or outputted from all the antennas.
Recently, in a high speed radio communication method such as LTE or WiMAX as above described, a Multiple Input Multiple Output (MIMO) technology in which the data is transmitted from plural antennas and received at the plural antennas has been adopted from the viewpoint of improving the frequency use efficiency. With this MIMO technology, it is required to transmit or receive different signals from the plural antennas, thereby to transmit or receive different signals from the plural antennas in the distributed antenna system.
As the background art in applying the MIMO technology to the distributed antenna system, there is JP-A-2010-068496 (patent document 1). This document discloses that “a power measuring part of the terminal measures the received power of a pilot signal transmitted from each antenna of the distributed antenna system for a long term. The terminal selects a predetermined number of antennas having strong received power as the communication antenna candidates and notifies the communication antenna candidates and the corresponding received powers to the radio base station apparatus. A channel estimation part of the terminal receives the communicable antenna and its granted antenna index information from the radio base station apparatus, and makes the channel estimation for the communicable antenna. To make the MIMO communication, it is required to decide a precoding matrix for making an operation on a send signal of the base station apparatus, whereby a precoding matrix index (PMI) of the communicable antenna is obtained based on the channel estimation of the terminal, and notified by communication to the radio base station apparatus using the antenna index information to control the phase rotation amount, the power ratio and so on for each antenna making the data communication.”
In applying the distributed antenna system to a macro cellular system, it may be possible to make the communication simultaneously between the antennas geographically dispersed over the wide area because of small mutual interference amount. JP-A-11-261474 (patent document 2) discloses a technique for measuring the coupling amount between the antennas in the distributed antenna system, and allowing the antennas having small mutual interference amount to communicate simultaneously on the same frequency channel to improve the repeated frequency use efficiency.
Also, in the MIMO communication, for accommodating plural users, there are a method for making the MIMO communication in one time slot with the Time Division Multiple Access (TDMA) by making the time division of the time slot for the terminal, and a method for making the multi-user MIMO communication for enabling the simultaneous communication among plural users simultaneously, whereby the examination result of each communication capacity is disclosed in non-patent document 1.